Influenza A viruses (IAV) are significant human pathogens causing yearly epidemics and occasional pandemics. Past pandemics have resulted in significant morbidity and mortality. The 1918 influenza pandemic was thought to have resulted in the death of at least 675,000 people in the U.S. and 40 million people worldwide. Pandemics in 1957 and 1968, while less severe, were also of major public health importance. A novel influenza A virus of swine origin became pandemic in 2009, causing the first pandemic in 41 years. In addition, annual epidemic influenza cases are also very significant resulting in approximately 36,000 deaths in the US annually. Given pre-existing cross-reactivity of the 2009 pandemic H1N1 virus with previously circulating H1N1 viruses, history suggests that the 2009 pandemic H1N1 influenza virus may face extinction unless it mutates to escape the already high global population immunity. The immune escape mechanisms potentially at its disposal include antigenic drift, antigenic shift via genetic reassortment, and intra-subtypic reassortment. Going back to the late 19th century, the evolutionary histories of past pandemic viruses are examined in an effort to better understand the nature and extent of the immune pressures faced by the 2009 pandemic virus in the immediate future. While human influenza viruses have often surprised us, available evidence leads to the hope that the current pandemic virus will continue to cause low or moderate mortality rates if it does not become extinct. It is important to develop clinical natural history studies to compare natural infections with the new H1N1 pandemic virus with those from seasonal influenza viruses. The major goal of this project is to evaluate the natural history of pandemic influenza virus infection in comparison to infection with seasonal influenza viruses in both immunocompromised and non-immunocompromised patients. We plan to ultimately recruit up to 1000 patients with pandemic or seasonal influenza virus infection. Recruitment occurred in both inpatient and outpatient settings at multiple sites including the NIH Clinical Center, Suburban Hospital and the Washington Hospital Center. Careful clinical evaluation, analysis of viruses collected from patients, and studies of the immune response of the patients is being performed. An ongoing effort is continuing to analyze the data collected over the past 3 years of this study and a report on these data is planned within the next 6 months. The 2009 influenza A(H1N1) pandemic called attention to the limited influenza treatment options available, especially in individuals at high risk of severe disease. Neuraminidase inhibitor-resistant seasonal H1N1 viruses have demonstrated the ability to transmit well despite early data indicating that resistance reduces viral fitness. 2009 H1N1 pandemic viruses have sporadically appeared containing resistance to neuraminidase inhibitors and the adamantanes, but the ability of these viruses to replicate, transmit, and cause disease in mammalian hosts has not been fully characterized. Two pre-treatment wild-type viruses and 2 post-treatment multi-drug-resistant viruses containing the neuraminidase H275Y mutation collected from immunocompromised patients infected with pandemic influenza H1N1 were tested for viral fitness, pathogenicity, and transmissibility in ferrets. The pretreatment wild-type viruses and post-treatment resistant viruses containing the H275Y mutation all demonstrated significant pathogenicity and equivalent viral fitness and transmissibility. The adamantane-resistant 2009 pandemic influenza A(H1N1) virus can develop the H275Y change in the neuraminidase gene conferring resistance to both oseltamivir and peramivir without any loss in fitness, transmissibility, or pathogenicity. This suggests that the dissemination of widespread multi-drug resistance similar to neuraminidase inhibitor resistance in seasonal H1N1 is a significant threat. During the past year viral seed stocks have been produced and manufacturing has begun of influenza A challenge virus strains for the purpose of human influenza challenge studies. A protocol has been developed, approvals sought, and resources put into place for such a study. A healthy volunteer screening study has already begun at the Clinical Center to begin identifying patients who will qualify and be available for these challenge studies during the next year.